Capsular polysaccharide A (PSA) of Bacteroides fragilis NCTC9343 has been reported to be an immunomodulator with therapeutic and preventative applications. U.S. Pat. Nos. 5,679,654 and 5,700,787; Tzianabos A O et al. (2000) J Biol Chem 275:6733-40. It was recently reported that in addition to PSA, B. fragilis NCTC9343 synthesizes at least seven other capsular polysaccharide (PSB-PSH). Krinos C M et al. (2001) Nature 414:555-558. It has also recently been reported that expression of seven of these eight capsular polysaccharides of B. fragilis are variable due to phase variation dictated by inversion of DNA segments containing the promoters of each of the polysaccharide biosynthesis loci. Krinos C M et al., supra. The fact that this strain synthesizes so many polysaccharides makes purification of PSA from this strain very laborious. This, coupled with the variable expression of PSA due to its phase variation, often results in a very low yield of PSA following extensive procedures for its purification. Scaled-up purification of PSA for preventative or therapeutic applications thus presents technical obstacles.